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ext4: optimize ext4_writepage() for attempted 4k delalloc writes 

In cases where the file system block size is the same as the page
size, and ext4_writepage() is asked to write out a page which is
either has the unwritten bit set in the extent tree, or which does not
yet have a block assigned due to delayed allocation, we can bail out
early and, unlocking the page earlier and avoiding a round trip
through ext4_bio_write_page() with the attendant calls to
set_page_writeback() and redirty_page_for_writeback().

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Theodore Ts'o 2015-10-03 10:49:23 -04:00
parent 937d7b84dc
commit cccd147a57
1 changed files with 12 additions and 1 deletions
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13
fs/ext4/inode.c
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@ -1815,11 +1815,22 @@ static int ext4_writepage(struct page *page,
* the page. But we may reach here when we do a journal commit via
* journal_submit_inode_data_buffers() and in that case we must write
* allocated buffers to achieve data=ordered mode guarantees.
*
* Also, if there is only one buffer per page (the fs block
* size == the page size), if one buffer needs block
* allocation or needs to modify the extent tree to clear the
* unwritten flag, we know that the page can't be written at
* all, so we might as well refuse the write immediately.
* Unfortunately if the block size != page size, we can't as
* easily detect this case using ext4_walk_page_buffers(), but
* for the extremely common case, this is an optimization that
* skips a useless round trip through ext4_bio_write_page().
*/
if (ext4_walk_page_buffers(NULL, page_bufs, 0, len, NULL,
ext4_bh_delay_or_unwritten)) {
redirty_page_for_writepage(wbc, page);
if (current->flags & PF_MEMALLOC) {
if ((current->flags & PF_MEMALLOC) ||
(inode->i_sb->s_blocksize == PAGE_CACHE_SIZE)) {
/*
* For memory cleaning there's no point in writing only
* some buffers. So just bail out. Warn if we came here